Radio frequency (RF) transceiver can be found in numerous applications, particularly in the field of wireless communications and radar sensors. In the automotive sector, there is an increasing demand for radar sensors used in so-called “adaptive cruise control” (ACC) or “radar cruise control” systems. Such systems may be used to automatically adjust the speed of an automobile so as to maintain a safe distance from other automobiles ahead.
Modern radar systems make use of highly integrated RF circuits, which may incorporate all core functions of an RF front-end of a radar transceiver in one single package (single chip transceiver). Such RF front-ends may include, inter alia, a voltage controlled oscillator (VCO), power amplifiers (PA), mixers, and analog-to-digital converters (ADC). However, not all of these components have to be necessarily in one single chip package. For example, ADCs may be integrated in a separate chip. Particularly in frequency-modulated continuous wave (FMCW) radar systems, the VCO is typically operated in a phase-locked loop (PLL). However, PLLs may be used in many other applications.
Radar applications used in automobiles are subject to various standards concerning road traffic safety, for example the functional safety standard ISO 26262 titled “Road vehicles—Functional safety”. To ensure the functional safety of a radar sensor, it is important to know whether the current state of the radar sensor allows a reliable distance and speed measurement. Particularly underestimating the true distance to a vehicle ahead and its speed may cause dangerous traffic situations.
In a radar system the operating state of the PLL used in the radar transceiver is a critical parameter. For a reliable measurement, the PLL should be in a locked state. That is, the PLL output signal is in phase with (i.e. locked to) a reference signal. If the PLL is in an “out-of-lock” state, any measurement results are unreliable, and therefore it may be important to know, whether the PLL is in a locked state or in an out-of-lock state. To improve functional safety, a lock/out-of-lock detector should provide a fast detection of an out-of lock state as well as precise detection of the locked state, which imposes a conflict of objectives on the circuit designer.